This invention relates, in general, to semiconductor processing, and more particularly, to a method of fabricating a defect-free semiconductor layer for semiconductor device manufacture therein.
In fabricating some semiconductor devices, an epitaxial layer is typically grown on a semiconductor substrate. Several layers of epitaxy can also be grown on the silicon substrate. A semiconductor substrate is a wafer which is sliced from a semiconductor crystal or ingot. The physical properties, resistivity and thickness, of the epitaxial layer can be better controlled in the vertical direction than the properties of the substrate. Better control allows for the optimization of certain device parameters. In addition, an epitaxial layer provides a means of controlling the doping beyond that available with diffusion or ion implantation.
Growing an epitaxial layer that is free of imperfections, defects, and impurities is required to manufacture high quality semiconductor devices, as well as to improve the yield. The quality of an epitaxial layer is determined by the quality of the substrate and the epitaxial growth process itself. Defects or impurities found in the substrate will propagate from the substrate to the epitaxial layer. Impurities and defects will also be introduced into the wafer substrate and epitaxial layer during the device fabrication and may increase at each step of the wafer processing.
A large amount of defects are formed when growing epitaxial silicon on a heavily doped substrate or buried layer, because the dopant source contains a high concentration of impurities such as heavy metals. The heavy metals can diffuse rapidly into the epitaxial silicon layer and form an epitaxial haze after thermal oxidation. Defects can also be formed by the primary dopant, such as boron. Crystal defects are formed at the epitaxial/substrate or epitaxial/buried layer interface because the boron atom, being smaller than the silicon atom, causes the silicon lattice to shrink. The buried layer is an active layer, thus defects must be prevented from forming in it, as well as the epitaxial layer.
Many defects are also generated during the fabrication of Silicon On Insulator (SOI) structures formed by Separation of silicon by Implantation of Oxygen (SIMOX) method. SIMOX structures are comprised of a substrate, an insulating layer on the substrate, and a top silicon layer on the insulating layer. The insulating layer is used to electrically isolate an integrated circuit fabricated in the top silicon layer from the substrate silicon. These types of structures are typically formed by implanting a heavy dose of oxygen into a bulk silicon wafer. The oxygen reacts with the silicon to form a silicon dioxide layer during a thermal anneal. Defects, such as threading dislocations, are generated at the silicon dioxide/top silicon layer interface and propagate into the top silicon layer during thermal anneal. These defects degrade device parameters and reduce the yield.
Several methods have been used to keep impurities/defects away from the active device regions fabricated in the epitaxial layer or in the upper region of a wafer substrate. One technique, called gettering, involves forming favorable trapping sites for impurities and mobile defects and then gettering them to the trapping sites. These favorable trapping sites, or gettering agents, are formed by several methods, such as: (1) the use of damage to the crystalline lattice by lapping, sandblasting, or ion implantation of argon; (2) forming a polycrystalline layer, such as silicon nitride, or polysilicon, on the back surface of the semiconductor wafer, where a strained layer at the substrate/polycrystalline interface acts as a trapping site for impurities/defects. These gettering agents, however, are not effective because they are not stable when subjected to high temperature operations. Furthermore, there is no method reported for the gettering of defects in the top silicon layer of a SIMOX wafer.
A method to reduce the number of defects in a silicon ingot and thus improve the strength of a silicon substrate is disclosed in U.S. Pat. No. 4,631,234, issued to Larrabee on Dec. 23, 1986. Here, a high concentration of germanium atoms is added to a silicon melt during the growth of the ingot. The germanium atoms are larger than the silicon atoms, thus block the propagation and multiplication of dislocations, which are generated at the edge of wafers during high temperature wafer processing, from spreading throughout the wafer substrate. Because dislocations degrade or weaken a substrate wafer, the strength of the substrate is improved by this invention. This method, however, would not significantly prevent the formation of defects in an epitaxial layer where the substrate is isolated from the epitaxial layer by an insulating layer or a buried layer. Some degree of gettering of impurities/defects from the epitaxial silicon is provided by the aforementioned invention because a strained layer, or misfit dislocations, may exist at the epitaxial/substrate interface. However, the degree of gettering action will be very minimal because a high density of misfit dislocations can not be produced. This is due to the fact that a single crystal silicon ingot can not be easily grown when the germanium concentration is doped higher than 2 percent. A high concentration of germanium in the melt leads to a constitutional supercooling at the crystal/melt interface which prevents the growth of single crystal silicon. A low concentration of germanium, lower than 2 percent, in the substrate will not be enough to alter the lattice parameters of the substrate to form an effective strained layer that will block the propagation of defects. In addition, the aforementioned invention can not be used to form a SIMOX structure having a built-in gettering agent, since this requires the formation of silicon on an insulating layer.
By now it should be appreciated that it would be advantageous to provide a method to reduce epitaxial defects in order to improve the yield and performance of semiconductor devices.
Accordingly, it is an object of the present invention to provide an improved method of forming a defect-free epitaxial semiconductor layer by using a stable gettering agent.
Another object of the present invention is to provide a method of using misfit dislocations to prevent the propagation of defects to a semiconductor layer and to getter defects and impurities throughout the entire wafer processing cycle.
A further object of the present invention is to provide a method of reducing defects in semiconductor wafers having a heavily doped boron buried layer by preventing the formation of misfit dislocations.
Yet another object of the present invention is to provide a method of growing a defect-free semiconductor layer on a Separation by Implantation of Oxygen (SIMOX) structure.
Still another object of the present invention is to provide a method of forming SIMOX structures with reduced defects in the top silicon layer.